The present invention relates to a projection exposure apparatus used, in a process of producing a semiconductor device such as an IC or LSI, to project a laser beam onto a substrate so as to form thereon a reduced image of a circuit pattern of a reticle (also called a mask).
In recent years, great advances have been made in the technology of producing semiconductor devices. With great advances in general semiconductor production technology, great advances have also been made in the microlithography technology. With the most advanced photolithography technology, it is possible to obtain high resolution in the sub-micron range. One possible method to further improve the resolution is to increase the NA (numerical aperture) of an optical system using exposure light with a fixed wavelength. Another method is to reduce the wavelength of the exposure light.
Various techniques based on the latter method have been proposed to improve the resolution using ultraviolet light with a small wavelength such as an excimer laser beam. The excimer laser apparatus, unlike conventional light sources such as a super-high pressure mercury lamp for emitting i-line light, is very large in size and weight, and thus it is installed separately from the main part of an exposure apparatus. Furthermore, in order to minimize the apparatus installation space in a clean room and also for the reason of safety, the excimer laser generator is installed, in most cases, in a room separated from a room in which the main part of the exposure apparatus is installed. For example, the excimer laser apparatus is installed in a room on a floor below the clean room or in a room which is on the same floor as the clean room but which is isolated from any other rooms.
The laser beam generated by the laser apparatus is transmitted to the exposure apparatus directly or through a plurality of light deflection elements such as mirrors, or through another type of optical path properly selected depending on the installation conditions. In any case, it is required that the optical path between the laser apparatus and the main part of the exposure apparatus should be tightly shielded. As is well known, a highly reliable safety system is required to ensure that no strong ultraviolet radiation such as an excimer laser beam leaks outward from the optical path. It is desirable that the inside of the optical path be purged with an inert gas such as nitrogen so that the optical elements in the optical path are prevented from being degraded. That is, it is required that the projection exposure apparatus using a laser beam should meet both requirements described above.
In view of the above, it is a general object of the present invention to solve the above problems. More specifically, it is an object of the invention to provide a technique which makes it possible to perform maintenance of the laser beam path of a projection exposure apparatus while ensuring complete safety, without causing degradation of optical elements, and at low cost.